Method for producing porous polyimide film, porous polyimide film and separator using same

ABSTRACT

A method for producing a porous polyimide film comprises: forming a first un-burned composite film wherein the first film is formed on a substrate using a first varnish that contains (A1) a polyamide acid or a polyimide and (B1) fine particles at a volume ratio (A1):(B1) of from 19:81 to 45:65; forming a second un-burned composite film wherein the second film is formed on the first film using a second varnish that contains (A2) a polyamide acid or a polyimide and (B2) fine particles at a volume ratio (A2):(B2) of from 20:80 to 50:50 and has a lower fine particle content ratio than the first varnish; burning wherein an un-burned composite film composed of the first film and the second film is burned, thereby obtaining a polyimide-fine particle composite film; and a fine particle removal step wherein the fine particles are removed from the polyimide-fine particle composite film.

TECHNICAL FIELD

The present invention relates to a method for producing a porouspolyimide film, a porous polyimide film, and a separator including thefilm.

BACKGROUND ART

In recent years, because of reductions in sizes of portable electronicdevices and development of hybrid automobiles, electric automobiles,etc. due to consideration of environmental problems such as increases inatmospheric pollution and carbon dioxide, secondary batteries havingexcellent characteristics, such as high efficiency, high output, highenergy density, and light weight are needed for such electronic devicesand electric automobiles. Various secondary batteries having suchcharacteristics have been developed and researched. Lithium secondarybatteries have been also variously researched in order to provide oneshaving such characteristics.

A chargeable and dischargeable lithium battery usually has a structurein which a space between a positive electrode (cathode) and a negativeelectrode (anode) is filled with a lithium salt, such as LiPF₆,dissolved in an electrolytic solution, for example, a non-aqueousorganic solvent. Lithium transition metal oxide is used as the positiveelectrode, and lithium or carbon (graphite) is mainly used as thenegative electrode. The electrolytic solution has a good ionicconductivity and a negligible electrical conductivity. During charging,lithium ions move from the positive electrode to the negative electrode,and during discharging, lithium ions move in the reverse direction.

The positive electrode and the negative electrode of the lithium batteryare separated from each other with a separator of a porous polymer filmand are formed into a structure preventing their direct electriccontact. Accordingly, the separator for a secondary battery is requiredto have various characteristics, such as film thickness (thinness),mechanical strength, ionic conductance (during containing of anelectrolytic solution), electric insulation, electrolytic solutionresistance, electrolytic solution-retaining property, and wettability.As the separators for secondary batteries having these properties, fineporous films made of polyolefins, such as polyethylene andpolypropylene, are generally used. These fine porous films have randompores at a porosity of about 35% to 40% and are widely used asseparators for lithium secondary batteries having negative electrodes ofcarbon.

However, these conventionally known separators are known to depositlithium metal on graphite negative electrodes due to repeated charge anddischarge cycles. Furthermore, the repetition of charge and discharge ofa battery is known to cause the growth of dendrite lithium, resulting ina short circuit of the battery and this problem needs to be solved(Patent Document 1). In contrast, providing uniform pores in a separatorin a regular manner is known to have an effect of improving theelectrical characteristics of a battery (Non-Patent Document 1).

Separately, it has been tried to use a polyimide having a highheat-resistance and high safety in the separator (Patent Documents 2 and3). However, the pores formed in the conventional polyimide film haveinsufficient uniformity and density.

-   Patent Document 1: Japanese Unexamined Patent Application    (Translation of PCT Application), Publication No. 2010-537387-   Patent Document 2: Japanese Unexamined Patent Application,    Publication No. 2011-111470-   Patent Document 3: Japanese Unexamined Patent Application,    Publication No. 2012-107144-   Non-Patent Document 1: Kazuhei Miyahara, and three others,    “Evaluation of fundamental properties of 3DOM PI separator and    production of metal lithium secondary battery”, The 53rd Battery    Symposium in Japan, proceedings, 3D21, p. 267 (2012)

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention has been made in view of the above circumstances,and an object thereof is to provide a method for producing a porouspolyimide film having uniform and dense pores on the surface in contactwith a surface of the negative electrode of a lithium-ion battery.

Means for Solving the Problems

The present inventors have found that a porous polyimide film havinguniform and dense pores can be produced by forming two different layerscomposed of fine particles and a polyamide acid or polyimide on asubstrate such that a layer on the substrate side contains a largeramount of the fine particles and that the battery performance can beimproved by disposing the film such that the surface having uniform anddense pores faces the negative electrode of lithium-ion battery, andhave arrived at the present invention.

A first aspect of the present invention relates to a method forproducing a porous polyimide film, the method comprising: a firstunburned composite film-forming step of forming a first unburnedcomposite film of a first varnish on a substrate, the first varnishcontaining a polyamide acid or polyimide (A1) and fine particles (B1) ata volume ratio (A1):(B1) of 19:81 to 45:65; a second unburned compositefilm-forming step of forming a second unburned composite film of asecond varnish on the first unburned composite film, the second varnishcontaining a polyamide acid or polyimide (A2) and fine particles (B2) ata volume ratio (A2):(B2) of 20:80 to 50:50 and having a fine particlecontent ratio lower than that of the first varnish; a burning step ofburning an unburned composite film composed of the first unburnedcomposite film and the second unburned composite film to prepare apolyimide-fine particle composite film; and a fine particle-removingstep of removing the fine particles from the polyimide-fine particlecomposite film.

A second aspect of the present invention relates to a porous polyimidefilm produced by the method according to the first aspect of the presentinvention.

A third aspect of the present invention relates to a separator composedof the porous polyimide film according to the second aspect of thepresent invention.

A fourth aspect of the present invention relates to a secondary batteryincluding an electrolytic solution and the separator according to thethird aspect disposed between a negative electrode and a positiveelectrode, wherein the surface of the separator disposed on thesubstrate side in the burning step of the first aspect is arranged onthe negative electrode side.

A fifth aspect of the present invention relates to a method forproducing an unburned composite film, the method comprising: a firstunburned composite film-forming step of forming a first unburnedcomposite film of a first varnish on a substrate, the first varnishcontaining a polyamide acid or polyimide (A1) and fine particles (B1) ata volume ratio (A1):(B1) of 19:81 to 45:65; and a second unburnedcomposite film-forming step of forming a second unburned composite filmof a second varnish on the first unburned composite film, the secondvarnish containing a polyamide acid or polyimide (A2) and fine particles(B2) at a volume ratio (A2):(B2) of 20:80 to 50:50 and having a fineparticle content ratio lower than that of the first varnish.

A sixth aspect of the present invention relates to a method forproducing a polyimide-fine particle composite film, the methodcomprising a burning step of burning the unburned composite filmproduced by the method according to the fourth aspect of the presentinvention.

Effects of the Invention

According to the present invention, it is possible to produce a porouspolyimide film having a large number of uniform and dense pores on itssurface in contact with a surface of the negative electrode of alithium-ion battery. Use of this film as the separator of a batteryallows easy movement of lithium ions and can thereby improve theelectrical characteristics of the battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph showing a surface of a porous polyimide filmproduced using a varnish containing fine particles at a volume fractionof 85%.

FIG. 2 is a photograph showing a surface of a porous polyimide filmproduced using a varnish containing fine particles at a volume fractionof 88%.

FIG. 3 is a photograph showing a surface of a porous polyimide filmproduced using a varnish containing fine particles at a volume fractionof 75%.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will now be described in detail,but the present invention is not limited to the following embodimentsand can be implemented with appropriate modifications within the purposeof the present invention.

Preparation of Varnish

In the present invention, two types of varnishes each containing apolyamide acid or polyimide, fine particles, and an organic solvent areprepared. Both varnishes are prepared through production of a polyamideacid or polyimide solution in which prescribed fine particles aredispersed.

The two types of varnishes are a first varnish containing a polyamideacid or polyimide (A1) and fine particles (B1) at a volume ratio(A1):(B1) of 19:81 to 45:65 and a second varnish containing a polyamideacid or polyimide (A2) and fine particles (B2) at a volume ratio(A2):(B2) of 20:80 to 50:50. In this regard, the second varnish shouldhave a fine particle content ratio lower than that of the first varnish.

The varnishes are each prepared by mixing an organic solvent in whichprescribed fine particles are dispersed in advance with a polyamide acidor polyimide at an appropriate ratio or by polymerizing a polyamide acidor polyimide in an organic solvent in which prescribed fine particlesare dispersed in advance. The fine particles may be any particles thatare insoluble in the organic solvent to be used in the varnishes and canbe selectively removed after film formation.

In the present invention, the volume ratio between (A1) and (B1) in thefirst varnish is the ratio between the volume determined by multiplyingthe mass of the polyamide acid or polyimide (A1) by its specific gravityand the volume of the fine particles (B1). The volume ratio between (A2)and (B2) in the second varnish is also determined as in above.

[Polyamide Acid]

The polyamide acid used in the present invention may be any one preparedby polymerizing appropriate tetracarboxylic dianhydride and diamine. Theamounts of the tetracarboxylic dianhydride and the diamine are notparticularly limited, and the amount of the diamine is preferably 0.50to 1.50 mol, more preferably 0.60 to 1.30 mol, and most preferably 0.70to 1.20 mol, based on 1 mol of the tetracarboxylic dianhydride.

The tetracarboxylic dianhydride can be appropriately selected fromtetracarboxylic dianhydrides that have been conventionally used as rawmaterials for synthesizing polyamide acids. The tetracarboxylicdianhydride may be an aromatic tetracarboxylic dianhydride or analiphatic tetracarboxylic dianhydride, but from the viewpoint of theheat resistance of the resulting polyimide resin, an aromatictetracarboxylic dianhydride is preferably used. The tetracarboxylicdianhydrides may be used in a combination of two or more thereof.

Preferred examples of the aromatic tetracarboxylic dianhydride includepyromellitic dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethanedianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride,bis(3,4-dicarboxyphenyl)methane dianhydride,3,3′,4,4′-biphenyltetracarboxylic dianhydride,2,3,3′,4′-biphenyltetracarboxylic dianhydride,2,2,6,6-biphenyltetracarboxylic dianhydride,2,2-bis(3,4-dicarboxyphenyl)propane dianhydride,2,2-bis(2,3-dicarboxyphenyl)propane dianhydride,2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride,2,2-bis(2,3-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride,3,3′,4,4′-benzophenonetetracarboxylic dianhydride,bis(3,4-dicarboxyphenyl)ether dianhydride, bis(2,3-dicarboxyphenyl)etherdianhydride, 2,2′,3,3′-benzophenonetetracarboxylic dianhydride,4,4-(p-phenylenedioxy)diphthalic dianhydride,4,4-(m-phenylenedioxy)diphthalic dianhydride,1,2,5,6-naphthalenetetracarboxylic dianhydride,1,4,5,8-naphthalenetetracarboxylic dianhydride,2,3,6,7-naphthalenetetracarboxylic dianhydride,1,2,3,4-benzenetetracarboxylic dianhydride,3,4,9,10-perylenetetracarboxylic dianhydride,2,3,6,7-anthracenetetracarboxylic dianhydride,1,2,7,8-phenanthrenetetracarboxylic dianhydride, 9,9-bisphthalicanhydride fluorene, and 3,3′,4,4′-diphenylsulfonetetracarboxylicdianhydride. Examples of the aliphatic tetracarboxylic dianhydrideinclude ethylenetetracarboxylic dianhydride, butanetetracarboxylicdianhydride, cyclopentanetetracarboxylic dianhydride,cyclohexanetetracarboxylic dianhydride,1,2,4,5-cyclohexanetetracarboxylic dianhydride, and1,2,3,4-cyclohexanetetracarboxylic dianhydride. Among thesetetracarboxylic dianhydrides, 3,3′,4,4′-biphenyltetracarboxylicdianhydride and pyromellitic dianhydride are preferred because of theirinexpensiveness and ready availability. These tetracarboxylicdianhydrides may be used alone or as a mixture of two or more thereof.

The diamine can be appropriately selected from diamines that have beenconventionally used as raw materials for synthesizing polyamide acids.The diamine may be an aromatic diamine or an aliphatic diamine, but fromthe viewpoint of the heat resistance of the resulting polyimide resin,an aromatic diamine is preferred. These diamines may be used in acombination of two or more thereof.

Examples of the aromatic diamine include diamino compounds having onephenyl group or about two to ten phenyl groups. Specifically, examplesof the aromatic diamine include phenylenediamines and their derivatives,diaminobiphenyl compounds and their derivatives, diaminodiphenylcompounds and their derivatives, diaminotriphenyl compounds and theirderivatives, diaminonaphthalenes and their derivatives,aminophenylaminoindanes and their derivatives, diaminotetraphenylcompounds and their derivatives, diaminohexaphenyl compounds and theirderivatives, and cardo-type fluorenediamine derivatives.

The phenylenediamines are, for example, m-phenylenediamine andp-phenylenediamine. The phenylenediamine derivatives are diamines towhich alkyl groups, such as a methyl group or an ethyl group, are bound,such as 2,4-diaminotoluene and 2,4-triphenylenediamine.

The diaminodiphenyl compounds are obtained by linkage of two aminophenylgroups at their phenyl groups. For example, the diaminodiphenylcompounds are 4,4′-diaminobiphenyl and4,4′-diamino-2,2′-bis(trifluoromethyl)biphenyl.

Alternatively, the diaminodiphenyl compounds are obtained by linkage oftwo aminophenyl groups at their phenyl groups via another group. Thelinkage is, for example, an ether linkage, a sulfonyl linkage, athioether linkage, a linkage of an alkylene or its derivative group, animino linkage, an azo linkage, a phosphine oxide linkage, an amidelinkage, or an ureylene linkage. The alkylene linkage is a linkage of analkylene having about 1 to 6 carbon atoms, and its derivative group isan alkylene group whose one or more hydrogen atoms have been replacedby, for example, halogen atoms.

Examples of the diaminodiphenyl compounds include 3,3′-diaminodiphenylether, 3,4′-diaminodiphenyl ether, 4,4′-diaminodiphenyl ether,3,3′-diaminodiphenyl sulfone, 3,4′-diaminodiphenyl sulfone,4,4′-diaminodiphenyl sulfone, 3,3′-diaminodiphenyl methane,3,4′-diaminodiphenyl methane, 4,4′-diaminodiphenyl methane,4,4′-diaminodiphenyl sulfide, 3,3′-diaminodiphenyl ketone,3,4′-diaminodiphenyl ketone, 2,2-bis(p-aminophenyl)propane,2,2′-bis(p-aminophenyl)hexafluoropropane,4-methyl-2,4-bis(p-aminophenyl)-1-pentene,4-methyl-2,4-bis(p-aminophenyl)-2-pentene, iminodianiline,4-methyl-2,4-bis(p-aminophenyl)pentane, bis(p-aminophenyl)phosphineoxide, 4,4′-diaminoazobenzene, 4,4′-diaminodiphenylurea,4,4′-diaminodiphenylamide, 1,4-bis(4-aminophenoxy)benzene,1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene,4,4′-bis(4-aminophenoxy)biphenyl, bis[4-(4-aminophenoxy)phenyl] sulfone,bis[4-(3-aminophenoxy)phenyl]sulfone, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, and 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane.

Among these, p-phenylenediamine, m-phenylenediamine, 2,4-diaminotoluene,and 4,4′-diaminodiphenylether are preferred because of theirinexpensiveness and ready availability.

The diaminotriphenyl compound is formed by linkage of two aminophenylgroups and one phenylene group, all of which are each linked throughanother group. The “another group” is selected from the same groups asin the diaminodiphenyl compounds. Examples of the diaminotriphenylcompounds include 1,3-bis(m-aminophenoxy)benzene,1,3-bis(p-aminophenoxy)benzene, and 1,4-bis(p-aminophenoxy)benzene.

Examples of the diaminonaphthalenes include 1,5-diaminonaphthalene and2,6-diaminonaphthalene.

Examples of the aminophenylaminoindanes include 5- or6-amino-1-(p-aminophenyl)-1,3,3-trimethylindane.

Examples of the diaminotetraphenyl compounds include4,4′-bis(p-aminophenoxy)biphenyl,2,2′-bis[p-(p′-aminophenoxy)phenyl]propane,2,2′-bis[p-(p′-aminophenoxy)biphenyl]propane, and2,2′-bis[p-(m-aminophenoxy)phenyl]benzophenone.

An example of the cardo-type fluorenediamine derivatives is9,9-bisanilinefluorene.

The aliphatic diamine has, for example, about 2 to 15 carbon atoms, andspecifically, examples thereof include pentamethylenediamine,hexamethylenediamine, and heptamethylenediamine.

The aliphatic diamine may be a compound having at least one substituentselected from the group consisting of halogen atoms and methyl, methoxy,cyano, and phenyl groups for hydrogen atoms of the diamine.

The polyamide acid to be used in the present invention may be producedby any method and, for example, can be produced by a known method, forexample, by reacting an acid and a diamine component in an organicsolvent.

The reaction of a tetracarboxylic dianhydride and a diamine is usuallyperformed in an organic solvent. The organic solvent to be used for thereaction of a tetracarboxylic dianhydride and a diamine may be anyorganic solvent that can dissolve the tetracarboxylic dianhydride andthe diamine without reacting with the tetracarboxylic dianhydride andthe diamine. The organic solvent may be a single solvent or a mixture oftwo or more solvents.

Examples of the organic solvent to be used for the reaction of atetracarboxylic dianhydride and a diamine include nitrogen-containingpolar solvents, such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide,N,N-diethylacetamide, N,N-dimethylformamide, N,N-diethylformamide,N-methylcaprolactam, and N,N,N′,N′-tetramethylurea; lactone polarsolvents, such as β-propiolactone, γ-butyrolactone, γ-valerolactone,δ-valerolactone, γ-caprolactone, and ε-caprolactone; dimethyl sulfoxide;acetonitrile; fatty acid esters, such as ethyl lactate and butyllactate; ethers, such as diethylene glycol dimethyl ether, diethyleneglycol diethyl ether, dioxane, tetrahydrofuran, methyl cellosolveacetate, and ethyl cellosolve acetate; and phenol solvents, such ascresols. These organic solvents may be used alone or as a mixture of twoor more thereof. The amount of the organic solvent is not particularlylimited but is desirably such that the content of the resultingpolyamide acid is 5% to 50% by mass.

Among these organic solvents, from the viewpoint of the solubility ofthe resulting polyamide acid, preferred are nitrogen-containing polarsolvents, such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide,N,N-diethylacetamide, N,N-dimethylformamide, N,N-diethylformamide,N-methylcaprolactam, and N,N,N′,N′-tetramethylurea.

The polymerization temperature is usually −10° C. to 120° C. andpreferably 5° C. to 30° C. The polymerization time varies depending onthe raw material composition and is usually 3 to 24 hours (hr). Theorganic solvent solution of the polyamide acid prepared under suchconditions preferably has an intrinsic viscosity of 1000 to 100000centipoises (cP), more preferably in a range of 5000 to 70000 cP.

Polyimide

The polyimide used in the present invention can be any known polyimide,without restricted by its structure and molecular weight, as long as thepolyimide is soluble in the organic solvent to be used in the varnish ofthe present invention. The side chain of the polyimide may have acondensable functional group, such as a carboxy group, or a functionalgroup enhancing the cross-linking reaction during burning.

In order to make the polyimide soluble in an organic solvent, it iseffective to use a monomer for introducing a flexible bend structureinto the main chain, for example, to use an aliphatic diamine, such asethylenediamine, hexamethylenediamine, 1,4-diaminocyclohexane,1,3-diaminocyclohexane, or 4,4′-diaminodicyclohexylmethane; an aromaticdiamine, such as 2-methyl-1,4-phenylenediamine, o-tolidine, m-tolidine,3,3′-dimethoxybenzidine, or 4,4′-diaminobenzanilide; apolyoxyalkylenediamine, such as polyoxyethylenediamine,polyoxypropylenediamine, or polyoxybutyrenediamine; apolysiloxanediamine; 2,3,3′,4′-oxydiphthalic anhydride,3,4,3′,4′-oxydiphthalic anhydride, or2,2-bis(4-hydroxyphenyl)propanedibenzoate-3,3′,4,4′-tetracarboxylicdianhydride. It is also effective to use a monomer containing afunctional group for improving the solubility in an organic solvent, forexample, to use a fluorinated diamine, such as2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl or2-trifluoromethyl-1,4-phenylenediamine. Furthermore, in addition to themonomer for improving the solubility of the polyimide, a monomer that ismentioned in the paragraph describing the polyamide acid may be usedwithin a range that does not inhibit the solubility.

The polyimide soluble in an organic solvent to be used in the presentinvention may be produced by any method and, for example, can beproduced by a known method of, for example, chemically imidizing orthermally imidizing a polyamide acid and dissolving the imidizedpolyamide in an organic solvent. Examples of such polyimides includealiphatic polyimide (full-aliphatic polyimides) and aromatic polyimides,and aromatic polyimides are preferred. The aromatic polyimide may be oneprepared by a thermal or chemical ring-closing reaction of a polyamideacid having repeating units represented by Formula (1) or one preparedby dissolving a polyimide having repeating units represented by Formula(2) in a solvent. In the formulae, Ar represents an aryl group.

The varnish of the present invention can be produced by mixing apolyamide acid or polyimide with an organic solvent in which fineparticles are dispersed in advance at an appropriate ratio, or bypolymerizing a tetracarboxylic dianhydride and a diamine into apolyamide acid in an organic solvent in which fine particles aredispersed in advance, or by further performing imidization into apolyimide. The final viscosity thereof is preferably adjusted to 300 to1500 cP and more preferably in a range of 400 to 700 cP. The varnishhaving a viscosity within this range can be formed into a uniform film.

When fine particles and a polyamide acid or polyimide are burned into apolyimide-fine particle composite film and when the material of the fineparticles is an inorganic material described below, the varnish maycontain the fine particles and the polyamide acid or polyimide such thatthe ratio of the fine particles to the polyimide is 2 to 6 (mass ratio).The ratio is more preferably 3 to 5 (mass ratio). When the material ofthe fine particles is an organic material described below, the fineparticles and the polyamide acid or polyimide may be mixed such that theratio of the fine particles to the polyimide is 1 to 3.5 (mass ratio).The ratio is more preferably 1.2 to 3 (mass ratio). Alternatively, thefine particles and the polyamide acid or polyimide may be mixed so as toprovide a polyimide-fine particle composite film having a volume ratioof the fine particles to the polyimide of 1.5 to 4.5. The ratio is morepreferably 1.8 to 3 (volume ratio). When a mass ratio or volume ratio ofthe fine particles to the polyimide is not lower than the lower limit ina polyimide-fine particle composite film, the film can have pores at anappropriate density as a separator. When the mass ratio or volume ratiois not higher than the upper limit, a film can be stably formed withoutcausing problems such as an increase in viscosity or cracking in thefilm.

Fine Particles

The material of the fine particles used in the present invention is notparticularly limited and may be any known material as long as thematerial is insoluble in the organic solvent used in the varnish and canbe removed later from the polyimide film. Examples of the inorganicmaterial include metal oxides, such as silica (silicon dioxide),titanium oxide, and alumina (Al₂O₃). Examples of the organic materialsinclude high-molecular-weight olefins (such as polypropylene andpolyethylene) and organic polymer fine particles, such as polystyrenes,epoxy resins, celluloses, polyvinyl alcohols, polyvinyl butyrals,polyesters, and polyethers.

Specifically, the fine particles are preferably, for example, colloidalsilica, in particular, monodisperse spherical silica particles which canform uniform pores.

The fine particles to be used in the present invention preferably have ahigh sphericity and a low particle diameter distribution index. Fineparticles satisfying these requirements show excellent dispersibility inthe varnish and can be used without causing aggregation with oneanother. As the fine particles, those having a particle diameter(average diameter) of, for example, 100 to 2000 nm can be used. The fineparticles satisfying these requirements can provide pores having uniformpore diameters to the porous film by removing the fine particles and canhomogenize the electric field to be applied to the separator.

The fine particles (B1) in the first varnish and the fine particles (B2)in the second varnish may be the same or different. In order to increasethe density of the pores on the side in contact with the substrate to behigher than that on the other side, the fine particles (B1) preferablyhave a particle diameter distribution index lower than or equal to thatof the fine particles (B2). Alternatively, the fine particles (B1)preferably have a sphericity lower than or equal to that of the fineparticles (B2). In addition, the particle diameter (average diameter) ofthe fine particles (B1) is preferably smaller than that of the fineparticles (B2). In particular, the fine particles (B1) preferably have aparticle diameter of 100 to 1000 nm (more preferably 100 to 600 nm), andthe fine particles (B2) preferably have a particle diameter of 500 to2000 nm (more preferably 700 to 2000 nm). The use of the fine particles(B1) having a particle diameter smaller than that of the fine particles(B2) can give pores at a high aperture proportion with a small variationin the aperture proportion on the surface of the polyimide film and canincrease the strength of the film compared to the case of using fineparticles having the same particle diameter as that of the fineparticles (B1) in the entire polyimide film.

The first varnish to be used in the present invention has a volume ratiobetween the polyamide acid or polyimide (A1) and the fine particles (B1)of 19:81 to 45:65. When the volume of the fine particles is 65 or morebased on 100 of the total volume of the first varnish, the particles areuniformly dispersed, and when the volume is 81 or less, the particlesare dispersed without causing aggregation with one another to allowuniform formation of pores on the surface of the polyimide film on thesubstrate side. When the volume ratio of the fine particles is withinthis range, mold releasability after formation of a film can be securedeven if the substrate to be used for forming an unburned composite filmis not provided with a mold release layer in advance.

The second varnish to be used in the present invention has a volumeratio between the polyamide acid or polyimide (A2) and the fineparticles (B2) of 20:80 to 50:50. When the volume of the fine particlesis 50 or more based on 100 of the total volume of the second varnish,the particles are uniformly dispersed without aggregating, and when thevolume is 80 or less, the particles are not aggregated and do not causecracking on the surface, resulting in stable formation of a porouspolyimide film having good electrical characteristics.

In the present invention, the content ratio of the fine particles in thesecond varnish is lower than that in the first varnish. By satisfyingthis requirement, even if the polyamide acid or polyimide contains alarge amount of the fine particles, the strength and flexibility of theunburned composite film, polyimide-fine particle composite film, andporous polyimide film can be secured. In addition, the lower contentratio of the fine particles in the layer can reduce the cost ofproducing the film.

In the present invention, the varnish may further contain a dispersantin addition to the fine particles, in order to uniformly disperse thefine particles in the varnish. The addition of the dispersant allowsfurther uniform mixing of the polyamide acid or polyimide with the fineparticles and further uniform dispersion of the fine particles in themolded or formed precursor film. As a result, dense apertures areprovided on the surface of the finally formed porous polyimide film, andthe front and rear surfaces can be efficiently communicated with eachother to improve the air permeability of the film.

The dispersant used in the present invention is not particularly limitedand may be any known one. Examples of the dispersant include, but notlimited to, anionic surfactants, such as salts of coconut fatty acid,salts of sulfonated castor oil, lauryl sulfate, polyoxyalkyleneallylphenyl ether sulfate, alkylbenzenesulfonic acid, alkylbenzenesulfonate, alkyldiphenyl ether disulfonate, alkylnaphthalene sulfonate,dialkyl sulfosuccinate, isopropyl phosphate, polyoxyethylene alkyl etherphosphate, and polyoxyethylene allylphenyl ether phosphate; cationicsurfactants, such as oleylamine acetate, lauryl pyridinium chloride,cetyl pyridinium chloride, lauryl trimethylammonium chloride, stearyltrimethylammonium chloride, behenyl trimethylammonium chloride, anddidecyl dimethylammonium chloride; amphoteric surfactants, such ascoconut alkyl dimethylamine oxide, fatty acid amide propyl dimethylamine oxide, alkyl polyaminoethyl glycine hydrochloride, amide betainesurfactant, alanine surfactant, and lauryl iminodipropionic acid;polyoxyalkylene primary alkyl ether or polyoxyalkylene secondary alkylether nonionic surfactants, such as polyoxyethylene octyl ether,polyoxyethylene decyl ether, polyoxyethylene lauryl ether,polyoxyethylene laurylamine, polyoxyethylene oleylamine, polyoxyethylenepolystyryl phenyl ether, and polyoxyalkylene polystyryl phenyl ether;other polyoxyalkylene nonionic surfactants, such as polyoxyethylenedilaurate, polyoxyethylene laurate, polyoxyethylenated castor oil,polyoxyethylenated hydrogenated castor oil, sorbitan laurate,polyoxyethylene sorbitan laurate, and fatty acid diethanolamide; fattyacid alkyl esters, such as octyl stearate and trimethylolpropanetridecanoate; and polyether polyols, such as polyoxyalkylene butylether, polyoxyalkylene oleyl ether, and trimethylol propanetris(polyoxyalkylene) ether. These dispersants may be used as a mixtureof two or more thereof.

[Production of Unburned Composite Film]

A method for forming an unburned composite film in the present inventionwill now be described. The first varnish is applied onto a substrate,such as a glass substrate, directly or with a mold release layerprovided in advance and is then dried under ordinary pressure or vacuumat 0° C. to 50° C., preferably under ordinary pressure at 10° C. to 30°C., to form a first unburned composite film. The step thus far isreferred to as the first unburned composite film-forming step.

Subsequently, the second varnish is applied onto the first unburnedcomposite film thus formed, and is similarly dried under ordinarypressure or vacuum at 0° C. to 50° C., preferably under ordinarypressure at 10° C. to 30° C. This step is referred to as the secondunburned composite film-forming step.

Subsequently, a burning step for forming a polyimide-fine particlecomposite film by burning the unburned composite films, the firstunburned composite film and the second unburned composite film, takesplace. The unburned composite films may be burned while being formed onthe substrate or may be peeled from the substrate prior to the burningstep.

In a case of peeling the unburned composite film from the substrate, thesubstrate provided with a mold release layer in advance can also be usedin order to further enhance the detachability of the film. In a case ofproviding a mold release layer in the substrate in advance, the moldrelease agent is applied onto the substrate and is dried or baked beforethe application of the varnish. The mold release agent used here may bea known mold release agent, such as an alkylphosphate ammoniumsalt-based or fluorine-based agent or silicon, without particularrestriction. When the dried unburned composite film is peeled from thesubstrate, a slight amount of the mold release agent remains on thesurface of the peeled unburned composite film and may lead todiscoloration during burning and adverse effects to the electricalcharacteristics, and the mold release agent should therefore be removedas much as possible.

Accordingly, in order to remove the mold release agent, the unburnedcomposite film peeled from the substrate is preferably washed with anorganic solvent. Any organic solvent that can dissolve the mold releaseagent without dissolving or swelling the unburned composite film can beused without particular restriction. For example, alcohols, such asmethyl alcohol, ethyl alcohol, and propyl alcohol, can be preferablyused. The washing method can be selected from known methods, such as amethod in which a film is immersed in a washing solution and is thentaken out or a method of shower washing. After washing, drying ispreferably performed. Here, a method of fixing ends of the unburnedcomposite film to a frame or the like made of stainless steel to therebyprevent deformation can be employed.

Alternatively, when a substrate is directly used without being providedwith a mold release layer in formation of the unburned composite film,the steps of forming the mold release layer and washing the unburnedcomposite film can be omitted.

[Production of Polyimide-Fine Particle Composite Film (Burning Step)]

The unburned composite film was heated as post-treatment (burning) intoa composite film (polyimide-fine particle composite film) composed of apolyimide and fine particles. The burning temperature varies dependingon the structure of the unburned composite film and the presence orabsence of a condensing agent and is preferably 120° C. to 375° C. andmore preferably 150° C. to 350° C. In a case of using an organicmaterial for the fine particles, the burning temperature must be set toa temperature lower than the thermal decomposition temperature of theorganic material. In the burning step, imidization is preferablycompleted.

The burning may be performed by, for example, a method of increasing thetemperature from room temperature to 375° C. over 3 hours and thenholding 375° C. for 20 minutes or a method of stepwise drying-thermalimidization by stepwise increasing the temperature by 50° C. from roomtemperature to 375° C. (holding the temperature of each step for 20minutes) and finally holding 375° C. for 20 minutes. When the unburnedcomposite film is peeled from the substrate once, an end of the unburnedcomposite film may be fixed to, for example, a frame made of SUSstainless steel to prevent deformation.

The thickness of the resulting polyimide-fine particle composite filmcan be determined by, for example, measuring the thicknesses of aplurality of positions with a micrometer or the like and averaging thethicknesses. Preferred average film thickness varies depending on thepurpose of the polyimide-fine particle composite film or the porouspolyimide film, however, is preferably 5 to 500 μm and more preferably10 to 100 μm, in the use as a separator for example.

[Porosification of Polyimide-Fine Particle Composite Film (FineParticle-Removing Step)]

The porous polyimide film can be produced with high reproducibility byselecting an appropriate method for removing the fine particles from thepolyimide-fine particle composite film.

For example, when silica is employed as the material of the fineparticles, the silica can be removed by treating the polyimide-fineparticle composite film with, for example, a low-concentration hydrogenfluoride solution to dissolve the silica.

Alternatively, an organic material may also selected as the material ofthe fine particles. Any organic material, which is decomposed at atemperature lower than polyimide, may be used. Examples of the fineparticles include resin particulates composed of linear polymers andknown depolymerizable polymers. The linear polymer usually has amolecular chain that is randomly cleaved during thermal decomposition;and the depolymerizable polymer is decomposed into a monomer duringthermal decomposition. Both of them are decomposed into a low molecularweight substance or to CO₂ and disappear from the polyimide film. Theresin fine particles to be used preferably have a decompositiontemperature of 200° C. to 320° C. and more preferably 230° C. to 260° C.A decomposition temperature of 200° C. or more allows formation of afilm even if the varnish contains a high boiling point solvent andbroadens the selection of burning conditions of the polyimide. Inaddition, a decomposition temperature of lower than 320° C. allows theresin fine particles to disappear without thermally damaging thepolyimide.

The total thickness of the porous polyimide film of the presentinvention is not particularly limited and is preferably 5 μm or more and500 μm or less, more preferably 10 μm or more and 100 μm or less, andmost preferably 10 μm or more and 30 μm or less. The above-mentionedthickness can be determined by, for example, measuring the thicknessesof a plurality of positions with a micrometer or the like and averagingthe thicknesses, as in the polyimide-fine particle composite film.

The layer formed from the first varnish preferably has a thickness of0.3 μm or more and 5 μm or less, preferably 0.4 μm or more and 4 μm orless, and more preferably 0.5 μm or more and 3 μm or less; and the layerformed from the first varnish preferably has a thickness of 4.3 μm ormore and 500 μm or less, preferably 4.5 μm or more and 99.7 μm or less,and more preferably 5 μm or more and 29.7 μm or less. The thickness ofeach laminar region can be calculated by averaging thicknesses at aplurality of positions in the first and second laminar regions of across section of a porous film observed with, for example, a scanningelectron microscope (SEM).

[Use of Porous Polyimide Film]

The porous polyimide film produced by the method of the presentinvention can be used as the separator of a lithium-ion battery, a fuelcell electrolyte film, a film for separating a gas or liquid, or a lowdielectric constant material. The porous polyimide film of the presentinvention can be used as the separator for a secondary battery, such asa nickel cadmium or nickel hydrogen battery or a lithium ion secondarybattery and is particularly preferably used as the porous separator fora lithium ion secondary battery. In particular, when the film is used asthe separator for a lithium-ion battery, the battery performance can beimproved by arranging the surface on the substrate side during theformation of the film on the negative electrode side of the lithium-ionbattery.

[Secondary Battery]

The secondary battery in the present invention is characterized in thatan electrolytic solution and the separator according to the third aspectare arranged between the negative electrode and the positive electrodeand that the surface of the separator arranged on the substrate side inthe burning step of the first aspect is arranged on the negativeelectrode side.

The secondary battery of the present invention may be of any type andmay have any configuration. The secondary battery is not particularlylimited to known secondary batteries such as nickel cadmium and nickelhydrogen batteries as well as a lithium ion secondary battery, as longas the battery has a configuration in which battery elements, a positiveelectrode, a separator, and a negative electrode laminated in this orderso as to satisfy the above-described requirements, are impregnated withan electrolytic solution and are encapsulated in an outer package.

The negative electrode of the secondary battery of the present inventioncan have a structure in which a negative electrode mixture composed of anegative electrode active material, a conductive auxiliary agent, and abinder is formed on a collector. For example, as the negative electrodeactive material, cadmium hydroxide can be used in nickel cadmiumbatteries, and a hydrogen-occlusion alloy can be used in nickel hydrogenbatteries. In lithium ion secondary batteries, a material that can beelectrochemically doped with lithium can be employed. Examples of suchactive materials include carbon materials, silicon, aluminum, tin, andWood's alloy.

Examples of the conductive auxiliary agent constituting the negativeelectrode include carbon materials such as acetylene black and Ketjenblack. The binder is composed of an organic polymer, and examplesthereof include polyvinylidene fluoride and carboxymethyl cellulose. Thecollector can be, for example, copper foil, stainless steel foil, ornickel foil.

The positive electrode can have a structure in which a positiveelectrode mixture composed of a positive electrode active material, aconductive auxiliary agent, and a binder is formed on a collector. Forexample, the positive electrode active material can be nickel hydroxidein nickel cadmium batteries and can be nickel hydroxide or nickeloxyhydroxide in nickel hydrogen batteries. Meanwhile, in lithium ionsecondary batteries, examples of the positive electrode active materialinclude lithium-containing transition metal oxides, specifically,LiCoO₂, LiNiO₂, LiMn_(0.5)Ni_(0.5)O₂, LiCo_(1/3)Ni_(1/3)Mn_(1/3)O₂,LiMn₂O₄, LiFePO₄, LiCo_(0.5)Ni_(0.5)O₂, and LiAl_(0.25)Ni_(0.75)O₂.Examples of the conductive auxiliary agent include carbon materials suchas acetylene black and Ketjen black. The binder is an organic polymer,and examples thereof include polyvinylidene fluoride. The collector canbe, for example, aluminum foil, stainless steel foil, or titanium foil.

The electrolytic solution in, for example, a nickel cadmium battery or anickel hydrogen battery is a potassium hydroxide solution. Theelectrolytic solution in a lithium ion secondary battery is composed bydissolving a lithium salt in a non-aqueous solvent. Examples of thelithium salt include LiPF₆, LiBF₄, and LiClO₄. Examples of thenon-aqueous solvent include propylene carbonate, ethylene carbonate,dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate,γ-butyrolactone, and vinylene carbonate. These solvents may be usedalone or as a mixture.

Examples of the outer package material include metal cans and aluminumlaminate packs. The shape of the battery is, for example, a rectangularshape, a cylindrical shape, or a coin shape; however, the separator ofthe present invention can be suitably applied to any shape.

The pores on the separator facing the negative electrode side of thebattery preferably have small diameters without variation in size andare preferably arranged densely to prevent charge concentration.Accordingly, when the surface of the porous polyimide film having beenarranged on the substrate side during the burning step is arranged onthe negative electrode side of a secondary battery, the dendrite growthof the metal from the negative electrode side due to repeated use of thesecondary battery can be suppressed compared to other cases. This ispreferable since the cycle characteristics of the secondary batteryimproved.

EXAMPLES

The present invention will now be more specifically described withreference to Examples, but the scope of the present invention is notlimited to the following examples.

In Examples and Comparative Examples, the following tetracarboxylicdianhydride, diamine, organic solvent, dispersant, and fine particleswere used.

Tetracarboxylic dianhydride: pyromellitic dianhydride

Diamine: 4,4′-diaminodiphenylether

Organic solvent: N,N-dimethylacetamide

Dispersant: polyoxyethylene secondary alkyl ether dispersant

Fine Particles:

Silica (1): silica having an average particle diameter of 300 nm

Silica (2): silica having an average particle diameter of 700 nm

Example 1

[Preparation of Varnish]

(1) First Varnish

The tetracarboxylic dianhydride (6.5 g), the diamine (6.7 g), and theorganic solvent (30 g) were put into a separable flask equipped with astirrer, a stirring blade, a reflux condenser, and a nitrogen gas inlettube.

Nitrogen was introduced into the flask through the nitrogen gas inlettube to provide a nitrogen atmosphere in the flask. Subsequently, thetetracarboxylic dianhydride and the diamine were reacted at 50° C. for20 hours by stirring the contents in the flask to prepare a polyamideacid solution. To the resulting polyamide acid solution was added 75 gof silica (1) as fine particles at a volume ratio between the polyamideacid and the fine particles of 22:78 (mass ratio: 15:85), followed bystirring to prepare a first varnish.

(2) Second Varnish

A second varnish at a volume ratio between the polyamide acid and thefine particles of 28:72 (mass ratio: 20:80) was prepared as in (1),except that 53 g of silica (2) as fine particles was added to theresulting polyamide acid solution.

[Formation of Polyimide-Fine Particle Composite Film]

The first varnish was applied onto a glass plate applied with a releaseagent with an applicator to form a film having a thickness of about 2μm. Subsequently, a film of the second varnish was formed on the film ofthe first varnish with an applicator. Pre-baking at 70° C. for 5 minuteswas performed to form an unburned composite film having a thickness of25 μm.

The unburned composite film was peeled off from the substrate, and therelease agent was then removed with ethanol. Thermal treatment wasperformed at 320° C. for 15 minutes to complete imidization.

[Formation of Porous Polyimide Film]

The polyimide-fine particle composite film was immersed in a 10% HFsolution to remove the fine particles contained in the film. The surfaceon the side to which the first varnish was applied, which would bebrought into contact with the negative electrode, was observed with anSEM. FIG. 1 shows the surface condition.

Comparative Example 1

A porous polyimide film was formed as in Example 1 except that the filmwas formed at once using only the second varnish of Example 1.

Comparative Example 2

A commercially available separator (Celgard 2000) was used.

Comparative Example 3

A porous polyimide film was formed as in Example 1 except that the filmwas formed at once using only the first varnish of Example 1. Theresulting porous polyimide film was very fragile and had poorhandleability.

Comparative Example 4

A porous polyimide film was formed as in Example 1 except that the filmwas formed at once using only the first varnish of Example 2 describedbelow. Although it was better than that in Comparative Example 3, theresulting porous polyimide film was fragile and had poor handleability.

Production of Coin Battery for Evaluation

In a coin-shaped outer container made of stainless steel having adiameter of 20 mm, a carbon negative electrode, a separator cut into acircle having a diameter of 14 mm, metal lithium cut into a circlehaving a diameter of 14 mm, and copper foil cut into a circle having adiameter of 14 mm and a thickness of 200 μm as a spacer were stacked inthis order; an electrolytic solution (1 mol·dm⁻³ of LiPF₆: a solutionmixture of ethylene carbonate/diethyl carbonate=1/1 (volume ratio)) wasadded to the container in several drops to such a degree that thesolution does not overflow from the container; and the container wascapped with a stainless steel cap via a polypropylene packing and wassealed with a caulking tool for coin batteries, to thereby produce abattery for evaluation of a negative electrode. In the production, theseparator of Example 1 was used such that the surface on the side towhich the first varnish was applied faced the negative electrode.Evaluation of charge-discharge characteristics of coin battery

The charge-discharge characteristics were evaluated by charging eachcoin battery for evaluation at a current density of 2.2 mAh (1C) up to4.1 V (CC-CV operation) and then discharging the battery at a currentdensity of 2.2 mAh (1C), 3C, or 5C down to 2.5 V (CC operation), in athermostatic chamber. Table 1 shows the results. In Table 1, the valuesshown in parentheses are the electrostatic capacity retention ratios (%)at a rate of 3C or 5C when the capacity at 1C is defined as 100%.

TABLE 1 Discharge capacity (mAh) 1 C 3 C 5 C Example 1 2.15 1.61 (74.8%)1.28 (59.5%) Comparative Example 1 2.05 1.41 (68.9%) 0.95 (46.3%)Comparative Example 2 2.08 1.32 (63.5%) 0.64 (30.8%)

Comparative Example 5

A porous polyimide film was formed as in Example 1 except that thevolume ratio between the polyamide acid and the fine particles of thefirst varnish was adjusted to 18:82 (mass ratio: 12:88). The surface onthe side to which the first varnish was applied, which would be broughtinto contact with the negative electrode, was observed with an SEM. FIG.2 shows the surface condition in Comparative Example 5.

Example 2

A porous polyimide film was formed as in Example 1 except that thevolume ratio between the polyamide acid and the fine particles of thefirst varnish was adjusted to 34:66 (mass ratio: 25:75) and that thevolume ratio between the polyamide acid and the fine particles of thesecond varnish was adjusted to 40:60 (mass ratio: 30:70). The surface onthe side to which the first varnish was applied, which would be broughtinto contact with the negative electrode, was observed with an SEM. FIG.3 shows the surface condition in Example 2.

It was demonstrated that a button battery according to Example 1 of thepresent invention shows a high capacity retention ratio at a high rateand has excellent electrical characteristics, compared to those ofComparative Examples. This is presumed to be caused by that a largernumber of pores can be formed in the surface on the negative electrodeside by forming two layers and to improve the Li ion mobility, comparedto those in Comparative Examples.

Comparison of the surface conditions of Examples 1 and 2 to that ofComparative Example 4 demonstrates that when the volume ratio of thefine particles is 82, agglomerates started to be generated to form largeholes. It was confirmed that although the volume ratio of the fineparticles in the first varnish of Example 2 was smaller than that inExample 1, the pores were uniformly formed. Considering both thatregular arrangement of uniform pores is recognized to be effective forimproving the electrical characteristics of the battery (see Non-PatentDocument 1) and that Example 1 gave the above-mentioned results, the useof the separator of the present invention produced using the firstvarnish containing the polyamide acid and the fine particles at a volumeratio of 19:81 to 45:65 probably can densely form pores having uniformsizes on the surface of the polyimide film and can improve theelectrical characteristics of the lithium-ion battery.

Example 3

A porous polyimide was formed as in Example 1 except that the thicknessof the film formed from the first varnish was about 1 μm and that thetotal thickness of the unburned composite film was about 20 μm.

Example 4

A porous polyimide was formed as in Example 3 except that the varnishwas prepared by using the dispersant in an amount of 10 parts by weightbased on 100 parts by weight of silica.

Comparative Example 6

A porous polyimide was formed as in Example 3 except that a monolayerprecursor film having a thickness of about 20 μm was formed using onlythe first varnish of Example 1 as the unburned composite film. Theaverage particle diameters on the front and rear surfaces of the porouspolyimide were the same. The resulting porous polyimide was very fragileand had poor handleability.

Comparative Example 7

A porous polyimide was formed as in Comparative Example 6 except thatthe second varnish of Example 1 was used.

The film characteristics of the porous polyimide films prepared abovewere evaluated, and the results are summarized in Table 2.

[Air Permeability]

A porous polyimide film having a thickness of about 25 μm (in which thefirst unburned composite film occupies about 1 μm) was cut into a 5-cmsquare sample. The time for 100 mL of air passing through the sample wasmeasured with a Gurley densometer (manufactured by Toyo Seiki Co., Ltd.)in accordance with JIS P 8117.

[Tensile Strength]

In order to evaluate the strength of a porous polyimide film, thetensile strength of the porous polyimide film was measured.

The porous polyimide films of Examples 3 and 4 and Comparative Examples6 and 7 were each cut into a 1 cm×5 cm strip sample. The stress (MPa) atthe time when this sample was broken was evaluated with RTC-1210ATENSILON (manufactured by ORIENTEC Co., Ltd.).

TABLE 2 Film Air Tensile thickness permeability stress Dispersant (μm)(sec) (MPa) Example 3 Absent 20 160 ≧7 Example 4 Present 20 65 ≧7Comparative Example 6 Absent 20 160 <1 Comparative Example 7 Absent 2016 ≧7

The porous polyimide film of Comparative Example 6 having a single-layerstructure showed low air permeability and also reduced strength of thefilm to make handling difficult. In the porous polyimide film ofComparative Example 7 similarly having a single-layer structure,although the film strength was improved to give a good handlingproperty, the value of the air permeability was low.

In contrast, the porous polyimide films of Examples 3 and 4 designed soas to satisfy the requirements of the present invention had improvedfilm strength to give excellent handling properties, while maintainingthe air permeability. Comparison of the film of Example 3 not using adispersant to the film of Example 4 using a dispersant demonstrates thatthe air permeability was slightly excellent in the film of Example 4using a dispersant.

Production of Coin Battery for Evaluation

In coin outer container made of stainless steel and having a diameter of20 mm, a carbon negative electrode, a separator of Example 3 or 4 orComparative Example 6 or 7 cut into a circle having a diameter of 14 mm,metal lithium cut into a circle having a diameter of 14 mm, and a spacerof copper foil cut into a circle having a diameter of 14 mm and athickness of 200 μm were stacked in this order; several drops of anelectrolytic solution (1 mol·dm⁻³ of LiPF₆: a solution mixture ofethylene carbonate/diethyl carbonate=1/1 (volume ratio)) were added tothe container so as not to overflow from the container; and thecontainer was capped with a stainless steel cap via polypropylenepacking and was sealed with a caulking tool for producing coin batteriesto produce a battery for evaluating a separator. On the occasion of theproduction, the separator was used such that the surface on the side towhich the first varnish was applied faced the negative electrode. Theresulting respective batteries were used as Examples B1 and B2 andComparative Examples B1 and B2. Evaluation of charge-dischargecharacteristics of coin battery

The charge-discharge characteristics were evaluated by charging eachcoin battery for evaluation at a current density of 2.2 mAh (1C) up to4.1 V (CC-CV operation) and then discharging the battery at a currentdensity of 2.2 mAh (1C) or 3C down to 2.5 V (CC operation), in athermostatic chamber. Table 3 shows the results. In Table 1, the valuesshown in parentheses are the electrostatic capacity retention ratios (%)at a rate of 3C when the capacity at 1C is defined as 100%.

Production of Monolayer Laminate Cell Battery for Evaluation

A positive electrode of 20 mm×20 mm and a separator of 20 mm×20 mm ofthe respective above-described Examples were placed in an aluminumlaminate outer container in this order, and an electrolytic solution(solvent: ethylene carbonate:ethyl methyl carbonate=3:7, electrolytesalt: LiPF₆ 1 mol/L) was added thereto. A negative electrode of 20 mm×20mm was placed in the container, and the battery case was sealed toobtain lithium ion secondary batteries of Examples B1 and B2 as well asComparative Examples B1 and B2. Here, the electrodes were anickel-cobalt-manganese ternary positive electrode and an artificialgraphite negative electrode, and were arranged such that the first layerwas in contact with the negative electrode.

Furthermore, monolayer laminate cell batteries were prepared similarlyto the above except that a commercially available polyethylene-based(PE-based) or cellulose-based separator was used. These batteries wereused as Comparative Examples B3 and B4. The PE-based separator beingused had a thickness of 20 μm, an air permeability of 270 seconds, and aporosity of 42%. The cellulose-based separator being used had athickness of 25 μm, an air permeability of 135 seconds, and a porosityof 70%.

Charge-Discharge Characteristics of Monolayer Laminate Cell Battery

Using the resulting lithium ion secondary battery, the potential changeby lithium occlusion was measured with a charge-discharge measuringapparatus. The battery was charged up to 4.2 V at a charging speed of0.2C at 25° C., and after a pause of 10 minutes, was then discharged ata discharging speed of 2C down to a voltage range of 2.7 V. After thedischarge, a pause of 10 minutes was taken. The battery was evaluatedfor the Ah utilization rate and Wh utilization rate (energy retentionratio) during this process.

Heat Resistance of Separator

The separator used in each battery was evaluated for the heat resistanceusing a soldering iron of about 250° C. according to the followingcriteria:

o: When a tip of the soldering iron is pressed against the center of afilm, a mark was left, but the film was not broken.

x: When a tip of the soldering iron is pressed against the center of afilm, the film was pierced.

Crushing Test of Monolayer Laminate Cell Battery

A crushing test was performed by charging a monolayer laminate cellbattery at a voltage of 4.2 V and then compressing the battery in aresting state in the direction vertical to the length direction with around bar having a diameter of 15.8 mm. The time when the voltagereduced was determined as the occurrence of internal short-circuit ofthe battery, and the battery was evaluated by the pressure at the timeof the occurrence of the internal short-circuit. The amount of reductionin voltage at 5 seconds after the time of the reduction of the voltagewas defined as ΔV (V). A higher value of the pressure is preferred, anda less reduction of the voltage is preferred.

TABLE 3 Results of evaluation of monolayer laminate cell battery Ab WhEvalution results utilization utilization of coin battery rate rateCrushing test Separator 1 C 3 C (%) vs (%) vs Heat Pressure Δ V used(mAh) (mAh) 0.2 C. 0.2 C. resistance (ton) (after 5 sec) Example Example3 2.1 1.6 88.0 84.2 ○ 0.56 0.26 B1 (74.7%) Example Example 4 2.2 1.788.1 84.3 ○ — — B2 (75.6%) Comparative Comparative 2.0 1.4 — — ○ — —Example Example 6 (70.2%) B1 Comparative Comparative 2.1 1.4 — — ○ — —Example Example 7 (64.3%) B2 Comparative PE — — 83.8 78.7 x — — ExampleB3 Comparative Cellulose — — 87.6 83.9 ○ 0.47 0.56 Example B4

The results of evaluation of the coin batteries shown in Table 2demonstrate that the use of the separator made of the porous polyimidefilm according to the present invention provides a higher capacityretention ratio than those of Comparative Examples 6 and 7. This ispresumed to be caused by that a larger number of pores can be formed onthe surface of the negative electrode side, while maintaining thehandleability, by forming two layers to improve the Li ion mobility.

In addition, the results of evaluation of monolayer laminate cellbatteries demonstrated that the use of the separator made of the porouspolyimide film according to the present invention provides: a capacityretention ratio and heat resistance which are superior to those of thePE-based separator; the capacity retention ratio which is also equal orsuperior to that of the cellulose-based separator; and superiority inthe crushing test.

The invention claimed is:
 1. A method for producing an unburnedcomposite film, the method comprising: a first unburned compositefilm-forming step of forming a first unburned composite film of a firstvarnish on a substrate, the first varnish containing a polyamide acid orpolyimide (A1) and fine particles (B1) at a volume ratio (A1):(B1) of19:81 to 45:65; and a second unburned composite film-forming step offorming a second unburned composite film of a second varnish on thefirst unburned composite film, the second varnish containing a polyamideacid or polyimide (A2) and fine particles (B2) at a volume ratio(A2):(B2) of 20:80 to 50:50 and having a fine particle content ratiolower than that of the first varnish.
 2. A method for producing apolyimide-fine particle composite film, the method comprising a burningstep of burning the unburned composite film produced by the methodaccording to claim 1.